1. Field of the Invention
This invention relates to a control device for variable speed electric motor which may be operated with a high efficiency by a solar battery as a drive source.
2. Description of the Prior Art
Recently, a pump to be driven by a solar battery as a power source has been used for irrigation and the like in a desert. A greatest merit of use of the solar battery is that a power distribution and transmission work is rendered easy and that the battery is not costly. Accordingly, if cost of the solar battery is reduced in the future, it is expected that such a pump will be rapidly spread. Constitution of a conventional device using the solar battery will be described below.
A conventional control device for such kind of variable speed electric motor is shown in FIG. 1, in which reference numeral 1 designates a variable speed electric motor (which will be hereinafter referred to as an electric motor) such as an irrigation motor (AC motor). A main circuit 2 serves to reversely convert a direct current to an alternating current so as to drive the electric motor 1. A solar battery 3 is constituted of a cell and the like, and generates a supply voltage according to a quantity of sunshine, etc. A control device 4 acts to drive the main circuit 2 so as to operate the electric motor 1 at variable speeds. The control device 4 is driven by a start switch 5 and controls the main circuit 2 at such a speed as set by a speed setting device 6.
In operation of the conventional device as mentioned above, when the start switch 5 is operated to command operation start, the control device 4 acts to on-off control a switching element included in the main circuit 2 according to a set value as set by the speed setting device 6. At this time, a direct current voltage is generated from the solar battery 3 according to a quantity of sunshine and the like, and the direct current is converted to an alternating current by the main circuit 2. If the alternating current voltage is greater than a voltage value permitting the electric motor 1 to be started, the electric motor 1 is started to drive the pump. In this case, the higher the rotational speed of the electric motor becomes, the greater the power becomes to improve an investment efficiency. However, if the pump is driven under full load (at a maximum rotational speed) with the small quantity of light, a voltage of the solar battery 3 is remarkably decreased to cause lack of output or stoppage of the control device 4. Generally, as shown in FIG. 2B, an open circuit voltage Voc of the solar battery 3 is little influenced by a quantity of light under no load, and always shows a high constant voltage value. Therefore, so long as a load is not actually connected to the battery, an output voltage is indefinite. For this reason, the motor is started to operate with a strength of sunshine chosen. However, if the quantity of light is small, the output voltage is widely decreased even by a slight output current.
Owing to the above-mentioned constitution of the conventional control device of the variable speed electric motor, it is difficult to seize the quantity of light before starting the electric motor. Further, if the quantity of light is not sufficient at starting, the output voltage is decreased just after starting, and accordingly a sufficient starting torque may not be provided for the electric motor, causing the electric motor not to be started. Furthermore, even when the quantity of light is changed during driving the electric motor, an output current of the solar battery cannot be controlled so that the output voltage of the solar battery may be maintained at values greater than a fixed value. If the quantity of light is insufficient, the output voltage of the solar battery is widely decreased to disadvantageously cause reduction in efficiency of the solar battery as well as impossibility of drive of the electric motor.